John, I second what Roman has said about being almost blind but 21 is a great age for a cat and the fact that he/she is still moving around (including the garden) is an indication that the cat's spirit is still in tact and that it hasn't 'given up'. Sounds like it's been well looked after to reach such a great age, too.
My cat was up til gone 3am playing with her Christmas toys - a remote control mouse and a general play area with a few gizmos. Needless to say, he has spent most of Boxing Day asleep - totally knackered!!!
